Remove 'single_price' field from supplier part

- Instead we will rely entirely on the SupplierPriceBreak model

InvenTree/build/migrations/0014_auto_20190518_1543.py

@@ -0,0 +1,19 @@
+# Generated by Django 2.2 on 2019-05-18 05:43
+
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('build', '0013_build_take_from'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='build',
+            name='take_from',
+            field=models.ForeignKey(blank=True, help_text='Select location to take stock from for this build (leave blank to take from any stock location)', null=True, on_delete=django.db.models.deletion.SET_NULL, related_name='sourcing_builds', to='stock.StockLocation'),
+        ),
+    ]